What is the significance of not outputting anything before using the header() function in PHP?

When using the `header()` function in PHP to set HTTP headers, it is important not to output anything (including whitespace) before calling this function. This is because headers must be sent before any actual content is output to the browser. If any output is sent before calling `header()`, PHP will throw an error like "Headers already sent". To solve this issue, ensure that there is no output (not even whitespace) before calling the `header()` function. 

<?php
// Ensure no output before setting headers
ob_start();

// Set the header
header("Location: https://www.example.com");

// Flush the output buffer
ob_end_flush();
?>

Keywords

header function output buffering HTTP headers content-type redirect

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